KANANGA, Congo (Reuters) - Two U.N. officials of American and Swedish nationality have been kidnapped in Congo's Kasai Central province, the Congolese government said on Monday.

Its statement said Michael Sharp, a U.S. citizen, and Zaida Catalan, of Swedish nationality, had "fallen into the hands of negative forces not yet identified", along with four Congolese they were with near the village of Ngombe.

Sharp and Catalan were among a U.N. panel of experts investigating the conflicts that have been simmering in Congo since the mid-1990s, when a civil war spawned dozens of armed groups and drew in half a dozen neighboring armies.

"The administrative and security services are working ... in concert with Monusco (the U.N. mission) to obtain the liberation of the kidnapped persons," the government statement signed by Information Minister Lambert Mende added.

A U.N. spokesman said they were missing and that U.N. peacekeepers were searching for them, without giving further details. The government statement gave no date for the incident.

The United Nations lists Catalan as Chilean.

Kasai Central province, in remote, heavily forested central Congo, has been riven by clashes between security forces and a local tribal militia called the Kamuina Nsapu since July.

At least 400 people have been killed and 200,000 have been displaced since the fighting broke out when police killed the militia's leader last August, the U.N. mission says.

U.N. High Commissioner for Human Rights Zeid Ra'ad al-Hussein said last week that three mass graves had been discovered in the area where the clashes are taking place.

Security across Congo has worsened since President Joseph Kabila failed to step down when his mandate expired in December.